The application of metal coatings to various surfaces by means of thermally sprayed molten metal particle is well known in the art. The application of anti-fouling metal coatings using the thermal spraying technique to marine structures, particularly hulls of boats and ships, is known, see Japanese Patent Document No. 56-33485 of April 1981. The process is also applicable generally to such exemplary structures as underwater pilings, power plant intake ducts, underwater energy conversion systems, buoys, off-shore drill platforms and the like where the fouling by marine growth interferes with or impedes the efficient operation of such apparatus.
Various systems have been devised for applying anti-fouling substances, typically copper and copper alloys, to marine surfaces, these include copper foils, panels or tiles which are adhered to hull surfaces. The most modern of these are paint and coating technologies which depend on uniform consumption of the binder and toxin and biocide and therefore are limited by the thickness or number of coatings applied. In the tile or foil methods, painstaking tailoring of individual panels or tiles to the complete hull surfaces has, in general, not been found acceptable by the marine trades. In Japanese Patent Document No. 56-33485 of April 1981, copper and copper alloy are thermally sprayed on a prepared resin bond coating, which may incorporate talcum, mica or fiberglass to provide antifouling protection for hulls, etc.
The present invention provides a distinct improvement over the art in that this invention includes, in a preferred embodiment, applying a curable adhesive layer onto the surface to be coated, spraying hollow glass, ceramic or carbon spheres or beads (and even phenolis beads or spheres) in the micronsize range (these microspheres are marketed under various trademarks such as Microballoons.TM.) onto the uncured adhesive layer, preferably so as to saturate the adhesive layer and then curing the adhesive layer. In some cases, the microsopheres can comprise a mix of glass and ceramic, or glass and carbon, or ceramic and carbon or glass, ceramic and carbon spheres, the ratios being tailored to the particular application. Thereafter the hollow beads and adhesive layer is abraded to rupture the hollow spheres and thermally sprayed with molten metal particles in one or more passes to form the metal layer. The adhesive layer can be a resin, preferably an epoxy which serves as the sealing layer, and firmly adheres the thermally sprayed metal coating. The mechanism is relatively simple in that the heavily filled resin layer is abraded by sanding or orit blasting sufficient to rupture, sheer and/or fracture the embedded hollow spheres. After the abrading process is completed, the surface is vacuumed or power-washed clean to remove the abraded material so that the surface now represents a porous surface with a matrix of large numbers of undercuts, nooks and crannies. The thermal spray process can employ either an oxygen/acetylene flame, electric arc to melt copper/nickel wire or combinations of these well known processes of spraying metal. The molten metal is atomized by compressed air into fine particles and propelled to the substrate. These particles are sufficiently hot and ductile to deform and embed themselves into the undercuts and recesses of the modified epoxy layer forming a strong mechanical bond. Sufficient passes build the deposit to a desired thickness. The sprayed molten metal, such as copper or copper based alloys for anti-fouling purposes flows into the undercuts, nooks and crannies and now becomes embedded into and mechanically locked to these pores and in this manner, the bond strength is mechanically fixed. The anti-fouling system includes a resin layer which could be a polyurethane a polyester or epoxy resin which serves three main functions: (1) provides an adhesive between the marine surface and a spray deposited copper or copper coating and (2) a seal layer to seal fine cracks in the gel coat of a fiberglass hull, for example, and (3) to prevent osmosis and a dielectric layer in the case of a steel hull to prevent electrolytic corrosion effects.
Spraying the hollow spheres or beads on the adhesive resin coating or layer provides a smooth uniform coating with less effort and process time, and the application of the resin layer, spraying with hollow spheres or beads, abrading or grit blasting and thermally spraying can all be easily automated. Spraying the spheres according to the invention can be on vertical as well as on overhead surfaces with equally advantageous results .